As consumer demand for increased capabilities in portable electronics and electric vehicles, it has become clear that new battery materials (anode, cathode and electrolyte) will need to be discovered to concurrently satisfy the weight, volume and power requirements of these emerging technologies.  At the cathode, there is a desire to create materials with capacities > 250 mAh/g and stability above 5 V vs Li/Li⁺[1].  At the anode, the targets are less well defined, though it is clear that graphite, which has been ubiquitous since the commercialization of Li-ion batteries, will not be sufficient. 

At the anode, an overwhelming amount of the attention from an R&D perspective has gone to the development of Si and Si-C composite materials [2].  Si is widely considered to be the most promising material because in of its high theoretical capacity, ~4000 mAh/g.  However, due to capacity mismatch penalties [3], the battery world is not yet in need of a material with such high capacity.  Additionally, Si has materials-related issues that need to be overcome.  For instance, metallurgical Si has very poor capacity retention due to extreme volumetric expansion and electrode pulverization.  Strategies to mitigate the structural issues with Si anodes include templating, voltage cutoff and Si-C composites, which despite being successful in improving capacity retention, also volumetrically and gravimetrically dilute the active material considerably.  Thus, most practical Si electrodes have true realized capacities less than 1000 mAh/g.  Lower achieved capacity, combined with the large volume of inactive material, have made it difficult for state-of-the-art Si to meet the USABC targets, Table 1.  Si-based anodes also typically have very low round-trip energy efficiencies, ~50% despite very high coulombic efficiency. 

At theoretical capacities ~1000 mAh/g, there are a considerable number of candidate anode materials, including transition metal oxides [4].  Metal oxide anodes typically are able to use nearly 100% of the active material [5], making their achievable energy density comparable to their theoretical capacity.  There is also the potential to decrease the volume of metal oxide-based anodes and a pathway exists for these materials to meet the USABC targets, shown in Table 1.  Metal oxides also have a safer lithiation potential that eliminates the problematic lithium plating process during charging.  However, the use of metal oxides has its own limitations including: low electronic conductivity, which leads to phase segregation of Li₂O and the base metal during charge-discharge, resulting in poor cycling stability. 

To improve the cycling stability of metal oxide anodes, our team has focused on creating nanostructured metal oxide-advanced carbon (graphene, carbon nanotubes, etc.) composite materials with high electronic conductivity. The high electronic conductivity of the graphene allows the metal oxides to create nano-sized domains [4], Figure 2, that allow for not only efficient but rapid chemical transformations.  Thus, our team has been able to increase the round-trip efficiency of metal oxide-graphene anodes to well over 70%, high capacity at 10C and excellent capacity retention over more than 500 cycles. 

In this presentation, our team will discuss the USABC target modeling with a particular focus on the interplay of battery characteristics with gravimetric and volumetric parameters.  We will also focus on half cell and full cell performance of MnO/MWCNT and NiO/RGO composite materials, including their rate capability and capacity retention over 100s of cycles. 

 References. 

1. A. Kraytsberg and Y. Ein-Eli, Advanced Energy Materials, 2 (2012) 922-939.
2. X. Su, Q. Wu, J. Li, X. Xiao, A. Lott, W. Lu, B. Sheldon and J. Wu, Advanced Energy Materials, 4 (2014) 1300882
3. M. Karulkar, J. Power Sources, 273 (2015) 1194-1201. 
4. N. Spinner, L. Zhang and W.E. Mustain, J. Mat. Chem. A, 2(6) (2014) 1627-1630
5. N.S. Spinner, A. Palmieri, N. Beauregard, L. Zhang, J. Campanella and W.E. Mustain, J. Power Sources, 276 (2015) 46-53.