Study of the Degradation Mechanisms of High-Performance Composite Anodes, Made of SiNi Nanoparticles (SiNiNPs) Coated By Carbon and CNT
Authors: Peled Emanuel , Golodnitsky Diana , Goldshtein Keren , Schneier Dan , Goor Meital , Freedman Kathrin 
Published: February 2020

In order to increase the energy density of the lithium battery, better anodes and cathodes are still required. Silicon has attracted much attention because its theoretical capacity is 4200mAhg−1, an order of magnitude greater than that of graphite. Unfortunately, silicon-based electrodes typically suffer from poor capacity retention. The capacity fade and large initial irreversible capacity of silicon anodes are caused by the extreme volume changes (about 320%) in silicon during lithiation/de-lithiation. This, in turn, causes a breakup of the electrode particles and electrical isolation of the active material. Volume expansion/contraction of silicon subjects its surface to a continuous reduction reaction of liquid electrolyte followed by the formation of a fresh solid-electrolyte interphase (SEI). Several degradation mechanisms are involved in the charge-discharge process of silicon anodes, including: 1) increase in SEI thickness and resistance, 2) reduction of solvents and salts, which leads to drying out of the electrolyte and to precipitation of a "secondary" porous SEI, 3) large increase in battery impedance and reduced power, 4) disintegration of the silicon particles, 5) loss of contact of the silicon particles to the current collector and 6) cracks in the electrode due to high volume expansion. Silicon nanostructures have the advantage of a shorter diffusion path for lithium species, which can improve the power performance of the battery.

In this work, in addition to the synthesis and characterization of novel high-capacity SiNiNP-based anodes, we focused on studying their degradation mechanisms. Simple synthetic routes have been developed for the preparation of SiNi-core/carbon-shell composite particles attached to carbon nanotubes. Si and Ni nanopowders were milled together in a ball mill, mixed with carbon nanotubes (CNT) and sucrose, and pyrolyzed at 1000oC. This synthesis was carried out with the aim of stabilization of the structure of the silicon active-anode material, inducing graphitization of the carbon shell and increasing the electron conductivity. We have been able to produce high loadings of up to 6mAh/cm2, low irreversible capacity (of the order of 17%), current efficiency greater than 99.5% and a fast charge–discharge rate (up to C/1.7). In half-cells, the loss of capacity after 175 cycles and at 0.05mA/cm2, is only about 16.5%, indicating that at least 83.5% of the SiNiNPs are still connected to the substrate. A full Jelly-Roll cell, made and tested by Tadiran using our SiNiNP anode of 1000mAh/ganode reversible capacity (three times that of the graphite anode) performed for 300 cycles. A 1000mAh/ganode anode enables significant reduction of the thickness and weight of the anode (by 66%), which leads to a 40% increase in the energy density over that of the common lithium-ion cell with graphite anode.

The thickness of a freshly formed SEI on lithium or on other substrate, such as nickel, copper, stainless steel and carbon is only a few nanometers (the tunneling range of electrons (Peled 1979)). Under open-circuit voltage conditions, and further exacerbation on cycling, the thickness and the resistance of the SEI grow with time. It was found that Rsei of the SiNiNP anode does not grow excessively with cycle number and reaches 40ohm.cm2 after 300 cycles. After prolonged cycling, a heavy precipitate of reduction products (secondary porous SEI) covers the anode. This precipitate, and not the breaking or disintegration of the anode, is the major reason for performance loss. The SEI consists of C, O, F and P. Water rinse of a disassembled cycled anode cleans the inorganic components (F, P and O) of the SEI. However, a thick organic residue (mainly polyolefins) remains and is assumed to limit the cell power after prolonged cycling. Under our test conditions, the most severe causes of capacity degradation are heavy precipitation of electrolyte reduction products between and on the SiNiNPs, and to a lesser extent, the increase in the SEI thickness, resistance and resistivity. The combined favorable properties - high capacity, high current efficiency, long cycle life, low irreversible capacity and high rate capability - make this anode a promising candidate for the next generation of lithium-ion batteries. The structure and compositional changes of the SiNiNPs and of the SEI will be reported.