Inhibiting Degradation Mechanisms of Conversion-Based Li-Ion Anodes through Nanostructured Hybrids
Authors: Ng Benjamin , Peng Xiong , Faegh Ehsan , Mustain William E. 
Published: February 2020

Electric vehicles (EVs) place extreme demands on Li-ion battery (LIB) packs. They must have high durability (>1000 cycles), high energy density (>500Wh kg-1), excellent safety characteristics and fast recharge times (~5-10 minutes).1,2 The future goal is to enable EVs that can not only travel locally, but also cross-country with an achievable driving range of 400-500 miles per charge. Unfortunately, state of the art LIBs are limited to gravimetric energy densities of ~250 Wh kg-1, which is equivalent to an EV with ~250 miles per charge. One method to increase the energy density, and hence achievable range of LIBs, is to develop active materials with higher energy density. At the anode, the most popular substitute for conventional graphite (372 mAh g-1) has been high-grade silicon (3600 mAh g-1). However, silicon experiences large volumetric expansion that leads to pulverization, contact loss, and poor capacity retention. Typically, the silicon is diluted with large quantities of carbon to distribute the strain from volumetric expansion, which significantly limits the effective capacity (to less than ¼ the theoretical capacity), energy density, and power density (due to mass transport limitations from thicker electrodes).
Therefore, there is a need to identify other possible replacements for the graphite anode with high achievable capacity (at least 800 mAh g-1), but reduced volumetric expansion during cycling. This opens the door for several possible chemistries that undergo conversion-type reactions with Li such as metal fluorides, nitrides, phosphides, hydrides, and oxides3,4 Of this list, metal oxides (MOs) are particularly attractive due to their relative ease in synthesis, low environmental impact and low cost. MOs are also intrinsically resistant to lithium plating and dendrite formation during fact charge (5-10 C) because of their higher reversible potential than graphite (~1 V vs. Li/Li+). Despite the positive properties of MOs, they have traditionally suffered from low cycle life – caused by their intrinsically low electronic conductivity that leads to phase separation – and low coulombic efficiency (95-98%). The low coulombic efficiency is caused by a degradation mechanism specific to MOs – reaction with the solid electrolyte interphase (SEI) to form higher oxidation states. This reaction, coupled with other degradation mechanisms (e.g. metal trapping, SEI growth) have limited the interest in metal oxides and their transition to full cells.
In this poster, we focus on two methods for increasing the cyclability of MO anode materials: 1) increasing inter-particle and intra-particle electronic conductivity; and 2) isolation of the MO active material through confinement. These are simultaneously achieved by forming composite anodes of MO nanoparticles and carbon – but with the carbon content limited to ≤ 10%. These new structures have enabled anodes with > 800 mAh g-1 capacity at 1C, > 450 mAh g-1 capacity at 5C, cycle life to > 2000 cycles at 1C with < 10% capacity fade and deployment of NiO-based anodes in stable full cells.

References

Tippmann, S., Walper, D., Balboa, L., Spier, B. & Bessler, W. G. Low-temperature charging of lithium-ion cells part I: Electrochemical modeling and experimental investigation of degradation behavior. J. Power Sources
252, 305–316 (2014).

Palmieri, A. et al. Improved Capacity Retention of Metal Oxide Anodes in Li-Ion Batteries: Increasing Intraparticle Electronic Conductivity through Na Inclusion in Mn 3 O 4. ChemElectroChem
5, 2059–2063 (2018).

Spinner, N. & Mustain, W. E. Investigation of metal oxide anode degradation in lithium-ion batteries via identical-location TEM. Mater. Chem. A
2, 1573–1992 (2014).

Spinner, N. S. et al. Influence of conductivity on the capacity retention of NiO anodes in Li-ion batteries. J. Power Sources
276, 46–53 (2015).