A Candidate Electrolyte for Li-Ion Batteries by chd21292


									 A Candidate Electrolyte for Li-Ion Batteries                                          LiBOB-based electrolyte are slightly better than cells
                                                                                       using LiPF6-based electrolyte.
         Fadhel Azeez and Peter S. Fedkiw*
Department of Chemical and Biomolecular Engineering                                    Acknowledgements
          North Carolina State University,                                                 The authors gratefully acknowledge Evonik,
              Raleigh, NC 27695-7905                                                   Hydro Quebec, and Chemetall for donated material and
                                                                                       Kuwait University for support of Fadhel Azeez.
Introduction                                                                                                                                         T oC
                                                                                                                 84           60           39            21            5         -10
           The need for compact, light-weight
                                                                                                                                    0.7 LiBOB/GBL+EA+EC (1:1:0)
                                                                                                                                  0.7 MM LiBOB/GBL:EA:EC(1:1:0)
rechargeable batteries offering high-energy densities

                                                                                    Conductivity (mS cm-1)
has become necessary in the 21st century especially for
portable electronic devices, hybrid electric vehicles,                                                  10
and       load     leveling      in      electric      power
generation/distribution. Among rechargeable batteries,
lithium-based systems are capable to fulfill these needs.
           The current state-of-art electrolyte for lithium-
ion batteries, which consists of LiPF6 dissolved in                                                                              Liquid
                                                                                                                                 10% R805
organic-carbonate solvents, has disadvantages in low-
                                                                                                                                 10% A200
and high-temperature environments. In order to                                                               1
improve the performance and enhance the safety of                                                                2.8             3.0        3.2             3.4            3.6         3.8
current lithium-ion batteries, a candidate electrolyte for                                                                                  1000/T(K-1)
these batteries is investigated. The electrolyte consists                              Fig 1. Conductivity vs. temperature for the liquid and
of lithium bis(oxalato)borate (LiBOB) salt dissolved in                                gel electrolytes.
a mixture of organic solvents consisting of γ-                                                  107
butyrolactone (GBL), ethyl acetate (EA), and ethylene                                                                  a)
carbonate (EC). In addition, we have added fumed                                                106
silica to this mixture to improve mechanical properties
                                                               G' and G"(Pa)

of the electrolyte. We find that LiBOB in a mixture of                                          105
GBL + EA + EC yields a technologically acceptable
conductivity, and upon adding fumed silica to LiBOB-
based electrolyte, a 3-D network structure (solid-like                                                                       0.2M LiBOB/GBL:EA:EC(1:1:0) + F.S
structure) is formed that improves the rheological                                                                                     A200  R805
                                                                                                103                              10%
properties of the electrolyte and safety of Li-ion
batteries without greatly affecting conductivity. In                                                                             20%
addition, we were able to construct a button cell with                                                                 0.1                   1                    10                   100
the gel electrolyte as the only separator (i.e., a polymer                                                                                 Frequency (rad/s)
film such as Celgard is not necessary to prevent
shorting of the electrodes.)                                                                            107

Experimental                                                                                            106
                                                                         G' and G" (Pa)

         The liquid electrolyte consists of LiBOB salt
dissolved in GBL+EA +EC. All solvents are dried over                                                    105
4 Å molecular sieves for at least one week. The gel
electrolytes were prepared by dispersing fumed silica                                                   104
                                                                                                                        1M LiBOB/GBL:EA:EC(1:1:0) + F.S
nanoparticles A200 or R805 (Evonik) into the liquid                                                                              A200 R805
electrolyte. The primary particle size of silica is about                                               103                10%
12 nm. The R805 fumed silica contains octyl surface                                                                        20%
group at 48% coverage and silanol surface group at                                                      102
52% coverage, and the A200 fumed silica contains only                                                                   0.1                      1                 10                   100
native silanol on the surface. The conductivity                                                                                             Frequency(rad/s)
measurements were performed using EG&G Model 273
potentiostat with a 5210 lock-in amplifier. Rheological                                Fig 2. Elastic modulus for gel electrolyte of
measurements were conducted using a TA AR2000                                          LiBOB/GBL:EA:EC of (1:1:0) (wt) a) 0.2 and b)1 M.
stress rheometer. Parallel plate geometry was used for
gel electrolyte. Cell discharge capacity measurements                                               120
were done using an Arbin model BT-2043 battery
                                                                   Discharge capacity (mAh/g)

cycler.                                                                                             100

Results and Discussion                                                                                                  LiMn2O4                   0.2C            25oC           3-4.2V
         The conductivity of the gel is close to that of                                                     60
liquid electrolyte, as illustrated in Fig. 1, which shows
the effect of adding fumed silica (two types of surface                                                      40                  1M LiPF6/EC:EMC (1:1)
chemistry) on the conductivity. Fig. 2 shows that the                                                                            1M LiBOB/GBL:EA:EC (1:1:1)
elastic modulus is independent of frequency. These                                                                               1M LiBOB/GBL:EA:EC (1:1:1) +10%A200
                                                                                                                                 1M LiBOB/GBL:EA:EC (1:1:1) +10%R805
rheological measurements indicate that adding fumed                                                          0
silica nano particles into LiBOB-based electrolyte                                                                 0                   5               10               15               20
forms a physical gel; however, the rheological moduli                                                                                                # of cycles
strongly depend on the surface group chemistry, EC
content, and salt concentration. The discharge capacity                                Fig 3. The discharge capacity of LiMn2O4/Li cell using
for LiMn2O4/Li cells using gel and liquid electrolytes is                              liquid and gel electrolytes.
shown in Fig. 3, which shows that the discharge
capacity for gel electrolyte is stable and close to
discharge capacity of cells using liquid electrolyte
(LiBOB-based electrolyte) and both of gel and liquid

 * Corresponding author.
   Tel.: 919-515-3572; Fax: 919-515-3465
   E-mail: fedkiw@eos.ncsu.edu

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