batteries Article Design Considerations for Fast Charging Lithium Ion Cells for NMC/MCMB Electrode Pairs William Yourey 1,2, * , Yanbao Fu 2 , Ning Li 2 , Vincent Battaglia 2 and Wei Tong 2   Citation: Yourey, W.; Fu, Y.; Li, N.; Battaglia, V.; Tong, W.Design Considerations for Fast Charging Lithium Ion Cells for NMC/MCMB Electrode Pairs. Batteries 2021, 7, 4. https://doi.org/10.3390/batteries70 10004 Received: 16 November 2020 Accepted: 30 December 2020 Published: 5 January 2021 Publisher’s Note: MDPI stays neu- tral with regard to jurisdictional clai- ms in published maps and institutio- nal affiliations. Copyright: © 2021 by the authors. Li- censee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and con- ditions of the Creative Commons At- tribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 College of Engineering, Penn State University Hazleton Campus, Hazleton, PA 18202, USA 2 Lawrence Berkeley National Laboratory, Energy Storage and Distributed Resources Division, Berkeley, CA 94720, USA; YanbaoFu@lbl.gov (Y.F.); NingLilbl@gmail.com (N.L.); VSBattaglia@lbl.gov (V.B.); Weitong@lbl.gov (W.T.) * Correspondence: wxy40@psu.edu Abstract: Lithium ion cells that can be quickly charged are of critical importance for the continued and accelerated penetration of electric vehicles (EV) into the consumer market. Considering this, the U.S. Department of Energy (DOE) has set a cell recharge time goal of 10–15 min. The following study provides an investigation into the effect of cell design, specifically negative to positive matching ratio (1.2:1 vs. 1.7:1) on fast charging performance. By using specific charging procedures based on negative electrode performance, as opposed to the industrial standard constant current constant voltage procedures, we show that the cells with a higher N:P ratio can be charged to ~16% higher capacity in the ten-minute time frame. Cells with a higher N:P ratio also show similar cycle life performance to those with a conventional N:P ratio, despite the fact that these cells experience a much higher irreversible capacity loss, leading to a lower reversible specific capacity. Keywords: Li-ion battery; graphite anode; cell design; negative to positive matching ratio; fast charging; reversible capacity ratio 1. Introduction Lithium ion cells continue to be the energy storage medium of choice for many electric vehicles (EV) currently in production [1]. For continued growth and further implemen- tation of electric vehicles into the transportation market, it is crucial that battery “refuel” times are reduced and become comparable to the time required to fill an internal com- bustion engine (ICE) vehicle fuel tank [2]. Current pathways to “refuel” battery powered vehicles are either quick cell replacement through exchange or swap stations [3], electrified roads [4], or fast charging lithium ion cells [5–7]. Much effort is being spent throughout industry and academia to develop fast charging batteries, and in regard to today’s currently produced cells and commercially available negative electrode active materials, it is widely accepted that the graphite negative electrode is a major factor, limiting fast charge capa- bility and affecting both cell performance and safety [8–14]. When attempting to quickly charge lithium ion cells containing a graphite negative electrode, polarization occurs and the negative electrode reaches voltages below 0.00 V vs. Li/Li + , allowing lithium metal plating to occur [15]. This occurs because most of the graphite capacity lies at or below 100 mV vs. Li/Li + [16], where a small polarization results in conditions favorable to lithium metal plating. Cell designs are possible using higher voltage negative electrode materials such as Li 4 Ti 5 O 12 (LTO), but there are drawbacks with these materials, namely high cost, lower specific capacity, and lower output cell voltage compared to graphite based mate- rials [17]. Based on these considerations, graphite is still the material of choice for most lithium ion cells in production today [18]. These cells are typically charged using constant current–constant voltage (CCCV) charge procedures, where cells are charged at relatively low constant currents until the cell reaches the maximum design voltage, at which point the cell is held at constant voltage until the current tapers to a specified cutoff amount. During Batteries 2021, 7, 4. https://doi.org/10.3390/batteries7010004 https://www.mdpi.com/journal/batteries