Electrochemical Degradation Caused By Mechanical Damage in All Solid State Battery Based on Al2O3 doped Li7La3Zr2O12

Abstract

All solid state lithium ion batteries consisting of solid electrodes and a Li ion conductive solid electrolyte have been expected to overcome the safety problem of present lithium ion batteries including flammable non-aqueous solvent . Solid solid interface contact between electrode and electrolyte layers also prevents the formation of the side reaction which is adverse for the lifespan of liquid batteries. However, the degradation of all solid state batteries(ASSBs) still exists even though it shows higher reliability compared to the traditional liquid batteries. Li7La3Zr2O12 solid electrolyte is one of the promising electrolytes for all solid state battery due to its high Li ion conductivity and stability against Li metal anode. For LiCoO2 cathode, decreasing the lithium concentration during charging process will lead to substantial repulsive forces between the O2 − ions in adjacent layers. It has been observed by Luo et al. that the maximum volume change rate LiCoO2 could be up to ~ 4.27% at the fully charging end. We suspect that the cathode interface formation in combination with the mechanical expansion ion provokes a contact loss between active material and solid electrolyte. We investigate the interface behavior at the cathode and demonstrate the important role of the interface between the active materials and the solid electrolyte for the battery performance. As solid electrolytes show quite limited plasticity and are unable to flow or infiltrate pores—in contrast to their liquid counterparts—mechanical contact loss due to irreversible shape changes of the LiCoO2 material is expected to have a significant contribution to the observed interfacial resistance. We reveal that the large volumetric change during the charge-discharge cycle and the resultant cracks are the primary mechanism of cyclic degradation of the ASSBs with rigid contact between cathode and solid electrolyte layers. Li7La3Zr2O12 bulk ceramic samples were fabricated by conventional solid-state reaction. The starting materials, Li2CO3, La(OH)3, γ-Al2O3 and ZrO2, were mixed by planetary ball-milling, and then calcinated at 800 °C for 10 h and 900°C for 10h. The calcinated powders were pressed into pellets and sintered at 1150 °C for 36 h in air. The cathode electrodes consist of 90 wt % active material and 10 wt % polyvinylidendifluoride solved in N-methyl pyrrolidone as a binder. No electrolyte material is added to purely focus on volume change behavior of the LiCoO2. Li foil was then attached on to the other side of the Li7La3Zr2O12 pellet as anode. All solid state battery with Li/Al2O3 added Li7La3Zr2O12 /LiCoO2 configuration was fabricated and its electrochemical properties were tested. The cycle life of the battery has been examined at temperature of 150 °C. All solid state cells (Li | Li7La3Zr2O12 | LiCoO2) were galvanostatically cycled within a potential window of 3.0–4.3 V vs Li/Li+ at 150 °C. The charge/discharge current density is set as 5 μA·cm− 2. All capacity was reported based on the mass of LiCoO2. The morphology of the electrode/electrolyte interface of the battery was observed using field emission-scanning electron microscopy (SEM). The theoretical electrochemical capacity of LiCoO2 is 137 mAh g−1, which corresponds to 0.5 Li per CoO2. The charge and discharge capacities of this battery were 12.6 mAh g−1, which corresponded to about 10% of the theoretical capacity. However, the corresponding discharge capacity is 2.7 mAh g−1 with columbic efficiency of 21%, indicating that most of lithium deintercalated from the cathode could not return to the lattice. The discharge capacity gradually decays and the capacity retention is 3% after 5 cycles. It can be seen that the first cycle is crucial for the overall capacity retention, as the most severe loss in capacity occurs in the first cycle. Such a strong capacity loss is not found when using LiCoO2 in a liquid electrolyte cell, suggesting that it is characteristic for the solid state environment. The attached figure presents the cross sectional SEM images of the interface between the cathode and the solid electrolyte. The results indicate that the destructive damage of cracks originated from the volume change of LiCoO2 layer is responsible for the degradation of cell during cycling. Figure 1