To improve the power performance of Li-ion batteries, it is important to understand the factors that limit the Li⁺ charge transfer kinetics. Li-ion batteries comprised of graphite anode and lithium cobalt oxide cathode in an electrolyte made of 1 M LiPF₆ in EC-DMC-DEC carbonate solvent mixtures could not deliver their room temperature capacity at a rate of C/2 at -30 and -40 ^oC [1]. When DEC was replaced by linear ester solvent, such as ethyl acetate (EA) or methyl butyrate (MB), the Li-ion batteries at -30 and -40 ^oC could deliver over 80% of their room temperature capacity at the same rate [2]. When the LiPF₆ salt is replaced by LiBOB in EC-EMC (1:1 wt. ratio) carbonate solvent mixture, the impedance of graphite-electrolyte interface measured using graphite/Li half cells in the electrolyte with LiBOB is 3 times that in the electrolyte with LiPF₆ [3]. These examples show that the electrolyte components play crucial roles in affecting Li⁺ charge transfer kinetics in Li-ion batteries.

The Li⁺ charge transfer process starts from the solvated Li⁺ in the electrolyte to the reception of an electron (e^-) from the electrode. This involves the de-solvation step of Li⁺ before entering into SEI and the diffusion step of Li⁺ through the SEI at the electrode and electrolyte interfaces before receiving an e^- from the electrode at the electrode and SEI interface. Abe et al. [4-6] believe that the Li⁺ desolvation is the rate limiting step, which is supported by the results from the investigation of the electrolytes of different solvent systems using LiClO₄ salt at HOPG, Li₄Ti₅O₁₂ and Li⁺ solid conductors interfaces. The question is whether the de-solvation as a limiting step can be extended to the cathode-electrolyte interface. Jow et al. [7] examined the Li⁺ charge transfer kinetics at the graphite anode-electrolyte interface (SEI) and LiFePO₄ (LFP) cathode-electrolyte interface (CEI) in a full cell, LFP/Gr, with Li as a reference electrode in 1 M LiPF₆ in EC-DMC-MB with VC as an additive. It is found that the activation energy (E_a) at the graphite-electrolyte is about 67 kJ mol-1, which is much higher than 33 kJ mol-1 found at the LFP-electrolyte interface. It is concluded that the electrodes and their associated electrode-electrolyte interfacial layers are controlling the Li⁺ charge transfer kinetics.

The impact of additives such as VC, LiBOB, and LiFSI, etc. on the impedance of the anode-electrolyte and the cathode-electrolyte interfaces in LiNiCoAlO₂/Gr cylindrical cells in 1.0 M LiPF₆ in EC-EMC-MP (20:20:60 vol %), where MP is methyl propionate, at low temperatures was studied by Jones et al. [8]. It is found that the additive impacts the impedance at the anode differently from that at the cathode. Further, we also found that the additives impact the E_a of Li⁺ charge transfer differently at the anode-electrolyte and the cathode-electrolyte interfaces. Different electrolyte components, including additives, result in different reduction and oxidation reactions during cell formation and cycling at the anode and cathode [9] and therefore, different and SEI and CEI layers. A discussion regarding the conditions in which the de-solvation step or the Li⁺ diffusion in either SEI or CEI layer is limiting the kinetics will also be provided.

Acknowledgement

Some work described here was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration (NASA).

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