Evaluation of Commercial Lithium-Ion Cells Based on Composite Positive Electrode for Plug-In Hybrid Electric Vehicle (PHEV) Applications

Abstract

Lithium-ion cells with composite positive electrodes are attractive and promising for EV and PHEV applications. For powertrain applications, the battery packs are required to have multiple-cell configurations, where some battery management is needed to protect cells from experiencing overcharging and overdischarging. Here, we show how to analyze the effect of slight overdischarge in a graphite || {Li x Mn 1/3 Ni 1/3 Co 1/3 O 2 + Li x Mn 2 O 4 } cell, when it was overdischarged to 2.0 V. We found a peculiar behavior at low voltages that is imputable to the composite nature of the positive electrode. Under certain circumstances, due to differences in kinetic limitations in each of the constituents in the composite electrode, although Li-ion batteries with graphite intercalation compounds (G) as negative electrodes (NE) and mechanically blended compositions nominally denoted as {Li x Mn a Ni b Co c O 2 (NMC) + Li x Mn 2 O 4 (LMO)} as composite positive electrodes (c-PE) could be attractive for plugin hybrid electric vehicle (PHEV) applications due to their ability of combining high power (rate capability) and high energy aspects. 1To acquire sufficient understanding of the performance of this type of cells, studies on various subjects, including aging process and degradation mechanism, quantification of cell-to-cell variations (cell variability) among a small quantity of cells; 2 the rate capability over a range of rates; 2 quantification of attributes from various degradation modes to capacity fading in a 2C cycle aging regime; 3 and, the amount of reversible and irreversible capacity fading at different temperatures in a temperature excursion; 4 have been reported in our previous studies. Here, a study with illustrations of computer model-generated schematics on a peculiar overdischarge phenomenon was conducted to understand cell behavior against overdischarge protection. It is useful to characterize such overdischarge events to understand the impacts of such variations on battery control and management during the operation of battery packs. The understanding shall shed some light on the particular events and their effects with regard to the consequences on reliability and safety of the battery system. Eventually, it would be desirable that various degrees of overdischarge be characterized, as similar overdischarge events may occur in cells in a string or battery pack where intrinsic cell variability, extrinsically induced imbalance, and variations in the operating conditions such as under temperature gradients may present in a system. Although this particular overdischarge phenomenon may only be relevant to this c-PE cell design and chemistry, this mechanistic study shall address the general concern of possible overdischarging events in composite electrode design and performance and their consequence in capacity fade with composite electrode designs.To receive a comprehensive understanding of this overdischarging behavior in the c-PE, one should refer to early studies of this family of materials, particularly th...

Durability and Reliability of Electric Vehicle Batteries under Electric Utility Grid Operations. Part 1: Cell-to-Cell Variations and Preliminary Testing

“…Computer simulations were performed using the 'Alawa toolbox (Hawai'i Natural Energy Institute (HNEI), Honolulu, HI, USA), a graphical interface for the mechanistic diagnosis model we proposed in 2012 [32,35]. This emulation approach, based on the matching of electrode half-cell data, has received experimental validation recently by other groups [36,37] and some example of its capabilities can be found in our previous work [38][39][40][41].…”

Mechanistic investigation of silicon-graphite/LiNi0.8Mn0.1Co0.1O2 commercial cells for non-intrusive diagnosis and prognosis

“…This phenomenon adds an additional complexity level to accurate diagnoses that was never investigated before. Although studies on blended PE were undertaken [33][34][35], herein, we introduce for the first time the emulation of a blended NE to properly address each active material from an individual perspective. We further expand in more detail the model construction, so it can be applied to either PE or NE blended materials.…”

Cell-balancing currents in parallel strings of a battery system

“…These cells were made by a brand name manufacturer. The cell chemistry has been studied in our laboratory in a series of investigations on subjects from cell variability [2], cycle aging and degradation mechanism [3], capacity fading in temperature excursions [4], a peculiar capacity fade phenomenon in the c-PE associated with overdischarge [5], to the methodology in determining state-of-charge (SOC) and state-of-health (SOH) of the cells and multi-cell strings [6,7]. Here, we discussed the transient aspects of parallel string configurations and the string balancing issues.…”