Cycle Life Assessment of Commercial Cells with Ni-Rich Cathode and Si-Graphite Anode: Are Advanced Batteries for Real?
Authors: Arora Shashank , Tammi Kari , Sainio Panu 
Published: December 2020

Growing interest in off-grid homes and long-range electric vehicles (EVs) powered by efficient energy storage systems is pushing the boundaries and driving the search for new materials for next-generation of Li-ion batteries (LIBs). For the state-of-the-art LIBs, pack level gravimetric and volumetric densities are smaller than the density values at materials level by up to four and two times, respectively, which makes the battery packs bulky [1, 2]. The next-gen LIB packs are required to be lightweight and extremely compact in order to maximize the usable space for the end-user [3].
Since, storage capacity of a battery is a function of its nominal capacity and operating voltage, impetus is on developing LIBs with higher upper cut-off voltages. One method is by replacing traditional cathodes with active materials that exhibit higher redox potentials vs. lithium reference electrode (Li+/Li) e.g. Li-rich layered oxides [4]. Silicon (Si) is a popular choice for high-energy LIB anodes. It is a naturally abundant mineral with the highest known theoretical capacity (4200 mAhg−1) and a low operating potential (< 0.5 V vs. Li/Li+). However, pure Si anodes display poor cyclability mainly due to volume changes in order of 100-300% which causes pulverization of active material and loss of electric contact with current collectors [5]. Nevertheless, commercial manufacturers have started to incorporate up to 12 wt.% of silicon in a graphite base to improve capacity of the resultant composite anode while benefitting from stability of a traditional carbon-based material [6].
Ensuring safety of LIBs is of prime importance, particularly for energy intensive applications such as EVs. It is also crucial to check systematically how the supposedly advanced batteries behave in different environments [7]. With this purpose in mind, comprehensive cycle life tests of 18650-type commercial cells with Ni-rich NCA cathode and Si-graphite composite anode were conducted in the present study. In addition to room-temperature environment, cell performance and degradation behavior were investigated at both the temperature extremes, 0 and 45 °C using the maximum (dis)charge current, specified by the manufacturer. Key aspects describing evolution of electrode morphology over cycle life e.g. porosity changes, SEI growth and elemental compositions were captured using impedance spectroscopy and physical characterization techniques. Comparisons were made to the degradation behavior of 18650-type commercial NMC/Si-graphite cell, recently tested by Smith and co-workers [8]. Counter-intuitively, the NCA cells analysed herein demonstrated better capacity retention and a significantly longer cycle life than the aforementioned NMC/Si-graphite cells in room temperature (1000 cycles Vs. 250 cycles); making the concept of “second-life applications” seem techno-economically viable.

References:

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