Amphi-redox Lithium Ion Battery

Abstract

Lithium ion batteries (LIBs) are conquering the market of energy storage devices due to their high energy density. However, higher energy densities are still required to meet energy demands of electric vehicles and large-scale energy storage devices used for smart grids. One direction enhancing the energy density is to use high-energy-density electrode materials as electroactive components. In this work, we (1) present high-capacity lithium manganese oxide spinel (LiMn2O4; LMO) in ~200 mAh g-1 as a cathode material, the capacity of which is two times as large as that of practical LMO and (2) realize the doubled capacity of LMO in LIB cells by the help of pre-lithiated graphite. Lithium manganese oxide spinel (LMO; LiMn2O4), which has been popularly used as a cathode material, has amphi-redox nature of releasing lattice lithium at ~4 V (n-type, equation 1 below) and accepting extra lithium at empty sites of ~3 V (p-type, equation 2 below) at 0 < x < 1: LiMn2O4 as an n-type (intrinsically reduced) cathode around 4 V with ~100 mAh g-1: LiMn3+Mn4+O4 = Li1-x Mn3+1-x Mn4+1+xO4 + x Li+ + x e- (1) LiMn2O4 as a p-type (intrinsically oxidized) cathode around 3 V with ~100 mAh g-1: LiMn3+Mn4+O4 + x Li+ + x e- = Li1+xMn3+1+x Mn4+1-xO4 (2) Only the 4 V reaction has been used for LIBs so that acceptable but not very high capacities around 100 mAh g-1 have been obtained from LMO. There are two reasons for why the 3 V reaction has not been used. First, the reaction at 3 V is seriously unstable partly due to Jahn-Teller distortion induced during the phase transition from the cubic at 4 V to the tetragonal at 3 V and partly due to low conductivity of the tetragonal phase. Second, additional lithium is required to reduce the Mn4+ of LiMn3+Mn4+O4 to use the 3 V reaction following the 4 V reaction in the charging processes of full cells. Third, Mn2+ ions largely dissolved from 3 + 4 V reaction aggravate prelithiated graphite by consuming lithium on Mn metal deposition. To solve the first problem, therefore, we improved the stability of the reaction at 3 V by enrobing LMO nanoparticles with a-few-layered graphene skin (LMO@Gn). The graphene skin obtained by high-energy ball milling guaranteed structural stability of the tetragonal phase and improved its electrical conductivity. To solve the second problem and to realize LIB cells based on 200 mAh g-1 LMO@Gn, we developed and optimized prelithiation process of anode materials and made a full cell configuration of LMO@Gn||LinC6 where LinC6 is lithiated graphite (n < 1). We let additional lithium released from the LinC6 fill out the empty tetragonal phase of LMO before the first charging process in lithium-ion batteries. Lastly, Mn metal deposition on prelithiated graphite was significantly alleviated by adopting multiple strategies composed of (1) alumina-coated separators to chelate Mn2+ ions, (2) vinylene carbonate (VC) as an additive in electrolyte, (3) heptamethyl disilazane (HEMDS) as a HF scavenger and (4) Na-carboxymethyl cellulose (CMC) as an ion-exchangeable binder in anode. The resultant cathode, Li2Mn2O4, showed its doubled capacity at ~200 mAh g-1 effectively in the LMO@Gn||LinC6 during cycling.