Heterogeneity of Graphite Lithiation in State‐of‐the‐Art Cylinder‐Type Li‐Ion Cells

Abstract

The two‐dimensional lithium distribution in the graphite anode was non‐destructively probed by spatially resolved neutron diffraction for a batch consisting of 34 different cylinder‐type (18650) Li‐ion batteries in fully charged state. The uniformity of the lithium distribution was quantified and correlated to the cell specifications/electrochemistry and to intrinsic cell parameters like electrode thickness, position of current collectors, etc. which were obtained by X‐ray micro‐computed tomography. Non‐uniformities in the lithiation state of the anode from a constant plateau have been observed for the majority of the studied cells. Their location corresponds to the positions of current tabs connecting the electrode stripes and areas of incomplete electrode coating at the beginning and the end of the electrode stripes. Four commonly used schemes of current lid connection were identified. Each of them displays its own effect on the uniformity of the lithiation at the anode and, therefore, variation of the intrinsic state‐of‐charge distribution and, most probably, the ageing behavior of the electrodes.

Methods—Spatially Resolved Diffraction Study of the Uniformity of a Li-Ion Pouch Cell

“…As described above, the c/a-ratio is correlated with the local SOC, see Figure 2. However, the state of lithiation in the cathode is less sensitive to the current distribution when compared to the graphite anode (25). The area of uniformly lithiated cathode exceeds that of the anode, and the edges with nonuniform lithiation are significantly narrower than those in the graphite anode.…”

Aging‐Driven Composition and Distribution Changes of Electrolyte and Graphite Anode in 18650‐Type Li‐Ion Batteries

“…[32] On the other hand, the temperature distribution at the extremum point (transition 1-2, at the vicinity of SOC 100 state), recently reported for a series of pouch bag cells [33] have been found very similar to the lithium distribution in the graphite anode [34,35] determined using X-ray diffraction radiography. Degree and uniformity of such lithium distribution has been found seriously affected by cell aging, [27,36,37] which can also take place for temperature surface profile.…”

“…Application of the distribution histogram analysis (similar to this reported in refs. [34,35,37]) enabled an unambiguous estimate of relevant distribution parameters. For example, the position of the peak in the histograms corresponds to the plateau concentration, and its width indicates the distribution quality while the fraction of the peak area over the total histogram area points on the degree of homogeneity.…”

Accelerating Battery Characterization Using Neutron and Synchrotron Techniques: Toward a Multi‐Modal and Multi‐Scale Standardized Experimental Workflow

“…Spatially resolved operando diffraction experiments are crucial to studying structural evolutions such as homogeneity and phase distribution in battery electrodes during operation,for example, phase redistribution in zinc–air batteries, [ 62 ] lithiation heterogeneities in graphite, [ 25 ] and silicon‐graphite, [ 63 ] long‐term cycling stability of high voltage spinel cathode versus graphite, [ 48,64 ] degradation mechanisms in Li–S batteries up to 35 cycles, [ 65 ] or NPD mappings. [ 66,67 ] Computed tomography (CT) methods are able to describe the macrostructure, interconnectivity, and electronic percolation in the electrodes with a spatial resolution from <100 nm to several micrometers. [ 68 ] The contrast in CT is obtained through absorption, phase shift or scattering methods, giving access to the crystal/nano structure (XRD‐CT, SAXS‐CT), oxidation state, and lithium‐ion bulk migration (NI, NCT), depending on the experimental methods employed.…”

“…[68] The contrast in CT is obtained through absorption, phase shift or scattering methods, giving access to the crystal/nano structure (XRD-CT, SAXS-CT), oxidation state, and lithium-ion bulk migration (NI, NCT), depending on the experimental methods employed. [66,69] μCT and nanoCT techniques provide invaluable insights into the mechanical and morphological degradation of electrodes, both of which play key roles in the performance decay for in state-of-the art lithium ion batteries as well as next generation technologies, for example, allsolid-state, [70,71] M-ion, [28,72] Li-O 2 [73,74] and Li-S batteries. [75] For instance, Na electrodeposits were observed in solid state batteries by μCT and found to increase interfacial resistance and hinder ion diffusion, resulting in performance decay.…”