XPSPES stability studies of iron silicate cathode - PDF by chd21292

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									                                                                                                                                      XPS/PES stability studies of iron silicate cathode materials
                                                                                                                                                     for up-scaled Li-ion batteries
                                                                                                                                                                                        David Ensling, Kristina Edström, Torbjörn Gustafsson and Josh Thomas
                                                            Ångström Advanced Battery Centre/Materials Chemistry, Uppsala University, Box 538, SE-751 21 Uppsala, Sweden.


                                                                                                                                Introduction                                                                                                                                                                                               XPS surface analysis of cycled Li2FeSiO4/C electrodes                                                                                                                                                                                                                                                      1     2     3
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   3,8

                                                                                                                                                                                                                                                                                                                                                                          A comparative study: LiTFSI vs. LiPF6




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         Potential vs. Li/Li (V)
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   3,6
  • Li2FeSiO4 is a recent addition to the group of interesting and potentially cheap cathode materials for




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         +
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   3,4

    large-scale lithium-ion battery applications. [1,2]                                                                                                                                                                                                                                                                                                            LiTFSI    C1s    LiPF6            LiTFSI F1s                                                                                                                                  LiPF6                                             3,2
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   3,0
                                                                                                                                                                                                                                                                                                                                                       C1s                                                  C1s
                                                                                                                                                                                                                                                                                                                                                                                                                                                    -CH




                                                                                                                                                                                                                                                                                                                             discharged
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    F1s                                              F1s                                                           2,8
  • The correlation between cycling performance and the chemical composition of the SEI formed on
                                                                                                                                                                                                                                                                                                                                                       LiFTSI                                               LiPF6
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    LiFTSI                                           LiPF6
                                                                                                                                                                                                                                                                                                                                                                                -CH                                                                                                                                                                                                                2,6
    the electrode has been probed by a combination of XPS and electrochemical measurements.                                                                                                                                                                                                                                                                                                                                                         C-C                                       -CF3
                                                                                                                                                                                                                                                                                                                                                         -CF3                   C-C                                                        -COH                                                                                                          LiF                                       2,4

                                                                                                                                                                                                                                                                                                                                                                            -COH                                                                                                             +binder                                                                                               2,2
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   2,0
  • Differences in solvent- and salt-stability have been observed for different electrolyte systems.                                                                                                                                                                                                                                                   a                                                    c                                                                         a                                               c

    An example of a comparative XPS study for LiTFSI- and LiPF6-based electrolytes is shown here.                                                                                                                                                                                                                                                                 -CO3                                                -CO3                                                                                LiF                                                                                      1,8
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     -0,1 0,0   0,1    0,2    0,3   0,4   0,5   0,6   0,7       0,8   0,9




                                                                                                                                                                                                                                                                                                                                           Intensity




                                                                                                                                                                                                                                                                                                                                                                                                Intensity




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         Intensity
                                                                                                                                                                                                                                                                                                                                                                                                                                                                        Intensity
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             x [Li2-xFeSiO 4]

                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      LiPF6                                      Discharge curves for the first three cycles of
                  Overview of salt systems studied and cycling performance




                                                                                                                                                                                                                                                                                                                             charged
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     +binder                                     Li2FeSiO4/C (55ºC, C/25) in LiTFSI EC:PC 1:1.
                                                                                                                                                                                                                                                                                                                                                                       -COOH                                                                                                                                                         +P-F-O                                      Minimal irreversible capacity observed during
                                                                                                                                                                                                        130
                                                             Cut-off voltage                                                                                                                                                    Solvents: EC : DEC 2:1                                  a- Charge Capacity                                             b                                                    d                          -COOH                                                                                                                                     the first cycles.
    Salt              Structure                                                                               Conc.                   Name                                                                                                                                              b- Discharge Capacity                                                                                                                                                                         b                                               d
                                                                   [V]
                                Li+                                                                                                                                                                     110                                                                                                     LiTFSI_1 a
                       O                    O                                                                                     Lithium
  LiTFSI      F3 C     S       N            S     CF3                                     2.0 – 3.7            1M         bis(trifluoromethane-                                                                                                                                                                 LiTFSI_1 b
                                                                                                                                                                                                                                                                                                                                                        294       291     288    285
                                                                                                                                                                                                                                                                                                                                                                 binding energy [eV]
                                                                                                                                                                                                                                                                                                                                                                                        282                  294               291     288    285
                                                                                                                                                                                                                                                                                                                                                                                                                              binding energy [eV]
                                                                                                                                                                                                                                                                                                                                                                                                                                                       282                          694       691 688 685          682               694      691 688 685          682
                                                                                                                              sulfonyl)imide                                                                                                                                                                                                                                                                                                                                                 binding energy [eV]                             binding energy [eV]




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    is
                       O                    O
                                                                                                                                                                                                                                                                                                                LiTFSI_2 a




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 ys
                                                                                                                                                                     Capacity [mAh/g]




                                                                                                                                                                                                                 90
                                                Li+
                                                                                                                                                                                                                                                                                                                                                                                                                                                     Si2p




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               ol
                                                                                                                                                                                                                                                                                                                               LiTFSI stable on cycling
                        F                                                                                                                                                                                                                                                                                       LiTFSI_2 b
                  F                F                                                                                             lithium
   LiPF6




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             dr
                                                                                          2.0 – 3.7            1M
                  F
                        P
                                    F                                                                                     hexafluorophosphate                                                                                                                                                                   LiPF6_1 a
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         Degradation of




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          Hy
                                                                                                                                                                                                                                            LiTFSI                                                                                                                                                                                          -SiO4                                            -SiO4
                                                                                                                                                                                                                                                                                                                                                                                                                                  Si2p                                 Si2p




                                                                                                                                                                                                                                                                                                                                                                                                             discharged
                                                                                                                                                                                                                                                                                   LiPF6
                        F                                                                                                                                                                                        70

                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         Li2-xFeSiO4?
                                                                                                                                                                                                                                                                                                                                                                                                                                  LiTFSI                               LiPF6

                                                                                                                                                                                                                                                                                                                                   Dynamic SEI coverage
                                                                                                                                                                                                                                                                                                                LiPF6_1 b
                                                                                                                                                        Specific Capacity [mAh/g]




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        --
                                     Li+
                                                                                                                                    Lithium
                                                                                                                                                                                                                                                                                                                                   for LiTFSI system due




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     <-
                  NC                 N                                                                                                                                                                                                                                                                          LiPF6_2 a
   LiTDI                                          CF3
                                                                                          2.0 – 4.0            1M              trifluoromethyl




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 HF
                                                                                                                                                                                                                 50                                                                                             LiPF6_2 b
                                                                                                                              dicyanoimidazole
                  NC
                                    N
                                                                                                                                                                                                                                                                                                                                   to polymerization of EC                                                                        a                                    c
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     Decomposition
                                                                                                                                   Lithium                                                                                                                                                                                         (Li-carboxylate type)
                               Li+

                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     of LiPF6 salt and




                                                                                                                                                                                                                                                                                                                                                                                                                      Intensity




                                                                                                                                                                                                                                                                                                                                                                                                                                                           Intensity
                               N

   LiPDI
              NC
                                            F2
                                            C         CF3
                                                                                          2.0 – 4.0            1M             pentafuoroethyl
                                                                                                                                                                                                                 30
                                                                                                                                                                                                                       0           8         16           24                  32   40            48
              NC               N                                                                                              dicyanoimidazole                                                                                                                 Cycle Number
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     LiF formation
                                                                                                                                                                                                                                                                                                                                          Reduced intensity of
                                                                                                                                                                                                                 140
              O                    Li+                O

                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         Cyclic voltammetry of the first five cycles of




                                                                                                                                                                                                                                                                                                                                                                                                             charged
                                                                                                                              Lithium bis-oxalato
                           O            O

   LiBOB                                                                                  2.0– 4.2            0.8 M                                                                                                                                                                                             LiTDI_1 a
                                                                                                                                                                                                                                                  LiTDI                                                                                   Li2-xFeSiO4 electrode
                               B
                                                                                                                                    borate
              O
                           O            O
                                                      O                                                                                                                                                                                                                                                         LiTDI_1 b                                                                                                                                                -Si-F                                      Small LiF peak                                       Li2FeSiO4/C (55ºC, C/25) in LiTFSI EC:PC 1:1.
                                                                                                                                                                                                                 120
                                                                                                                                                                                                                                                                                                                LiTDI_2 a                 emissions due to                                                                        b                                    d
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    when using LiTFSI                                    The undoped Li2FeSiO4 material undergoes a
Safe transfer from cell to XPS analysis                                                                                                                                                                          100
                                                                                                                                                                                                                                                                                                                LiTDI_2 b

                                                                                                                                                                                                                                                                                                                LiPDI_1 a
                                                                                                                                                                                                                                                                                                                                          thicker surface layer
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         transition during the first cycles. The system
                                                                                                                                                                                                                                                                                                                                                                                                                                  107 105 103 101 99                   107 105 103 101 99
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         stabilizes after the 3rd cycle.
                                                                                                                                                                                        C a pa c ity [m A h/g]




           Ar glove-box                                     Transfer vessel                                    PES analysis                                                                                                                                                        LiPDI                        LiPDI_1 b                                                                                                            binding energy [eV]                  binding energy [eV]
                                                                                                                                                                                                                                                                                                                LiPDI_2 a
                                                                                                                                                                                                                  80
                                                                                                                                                                                                                                                                                                                                                                                                                  Ensling et al., J. Mater. Chem. (2008) (in press)
                                                                                                                    hn                                                                                                                                                                                          LiPDI_2 b

                                                                                                                          e-                                                                                                                                                                                    LiBOB_1 a
                                                                                            S                            S                                                                                        60
                                                                                                                                                                                                                                                          LiBOB                                                 LiBOB_1 b

                                                                                                                                                                                                                                                                                                                LiBOB_2 a
                                                                                                                                                                                                                                                                                                                                                                SEI model                                                                                                                       Conclusion
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                Conclusions                                                                                                                 SEI model
References: [1] A. Nytén, A. Abouimrane, M. Armand, T. Gustafsson and J.O. Thomas,

                                                                                                                                                                                                                                                                                                                              LiTFSI                                                                                                          LiPF6-based electrolyte                                                                                                                                    LiPF6
                                                                                                                                                                                                                                                                                                                LiBOB_2 b
                      Electrochem. Comm., 7 (2005) 156.                                                                                                                                                           40
                                                                                                                                                                                                                                                                                                                                                                                       PEO                                                                                                                                                                                                                                                LiPF6/LixPFy/LixPOyFz
            [2] A. Nytén, S. Kamali, L. Häggström, T. Gustafsson and J.O. Thomas,                                                                                                                                      0           8         16           24                  32   40            48
                                                                                                                                                                                                                                                                                                                                                                                       LiTFSI                                                                                                                                                                                                                                             PEO
                                                                                                                                                                                                                                                                                                                                                                                                                                           • forms thin surface layer with small amounts of
                                                                                                                                                                                                                                                               Cycle Number

                      J. Mater. Chem. 16 (2006) 2266.                                                                                                                                                                                              Cycle number                                                                                                                        Li2CO3                                                                                                                                                                                                                                             LiF
                                                                                                                                                                                                                                                                                                                                                                                       C/Li2FeSiO4
                                                                                                                                                                                                                                                                                                                                                                                                                                             carbon-based reaction products
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          C/Li2FeSiO4
                                                          Electrochemical performance of Li2FeSiO4 electrodes                                                                                                                                                                                                                                                                                                                              • LiPF6 salt is unstable - decomposing to LixPFy,
                                                                                                                                                                                                                                                                                                                             EC (C3O3H4)                                Polymer, e.g., PEO
                                                                                            3,8
                                                                                                  a
                                                                                                                                              3,8
                                                                                                                                                    b                                                                                                                                                                                                                                                                                        LixPOyFz and LiF                                                                                                                                            LiPF6 / LixPFy / LixPOyFz
                                                                                            3,6                                               3,6
                                                                                                                                                                                                                                                                                                                                 LiTFSI /
                                                                                            3,4                                               3,4                                                                                                                                                                                TFSI-                                     Li2CO3                                                          • indications of Li2FeSiO4 degradation through reaction                                                                                                                                        Free     PEO
                                                               Potential vs. Li /Li (V)




                                                                                            3,2                                               3,2                                                                                                          First-cycle charge and discharge                                                                                                                                                  with HF (formed in hydrolysis of LiPF6)                                                                                                                                  LiF                 surface?
                                                              +




                                                                                            3,0                                               3,0                                                                                                          of Li2FeSiO4/C electrodes (55ºC,
                                                              o




                                                                                                                                                                                                                                                           C/20) in: a) LiTFSI EC:PC (1:1)
                                                                                            2,8                                               2,8
                                                                                                                                                                                                                                                           and b) LiPF6 EC:PC (1:1)                                                                                                                                                         LiTFSI-based electrolyte
                                                                                            2,6                                               2,6

                                                                                            2,4                                               2,4                                                                                                                                                                                                                                                                                          • polymerisation of the EC/DEC solvents; the salt is
                                                                                            2,2
                                                                                                               LiPF6                          2,2
                                                                                                                                                                                                                           LiTFSI                                                                                                                                                                                                            stable
                                                                                            2,0

                                                                                                  0
                                                                                                      first charge: 93 mAh/g
                                                                                                         20    40        60      80    100
                                                                                                                                              2,0   first charge: 100 mAh/g
                                                                                                                                                    0                                            20                        40    60    80    100
                                                                                                                                                                                                                                                                                                                              Solvent decomposition                                                                                        • dynamic - even reversible - SEI coverage depending                                                                                                    Salt/electrode degradation
                                                                                                      Specific Capacity (mAhg )
                                                                                                                                       -1
                                                                                                                                                             Specific Capacity (mAhg )
                                                                                                                                                                                                                                             -1                                                                                                                                                                                              on state-of-charge (SOC)

								
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