Why High-Nickel NCM Cathode Materials Show “M”-Type Swelling Behavior

1. What Is “M”-Type Swelling Behavior in Lithium-Ion Battery Cathode Materials?

Lithium-ion batteries experience structural swelling and contraction during delithiation and lithium intercalation. For anode materials — whether intercalated lithium in graphite or alloyed lithium in silicon-based anodes — the common feature is a clear volume expansion on lithium insertion and a clear shrinkage on lithium extraction, consistent with conventional expectations. During pouch-cell swelling testing, however, cells built with some cathode systems — particularly high-nickel ternary (NCM) materials — shift from expansion to contraction at the end of charging, then expand briefly before shrinking again at the start of discharge. This is the “M”-type swelling behavior, and it is most likely caused by the cathode electrode, which is why cathode-side swelling behavior deserves closer attention.

2. How Do NCM111 and NCM622 Cathode Materials Differ in Swelling Behavior?

Two ternary cathode materials with different nickel contents, NCM111 and NCM622, were assembled into button-type full batteries (conventional graphite anodes in both cases) to measure swelling thickness through charge-discharge cycling. Testing used IEST silicon-based anode swelling in-situ rapid screening system (RSS1400, Figure 1(a)), with swelling thickness results shown in Figure 1(b).

For the button-type full battery using NCM111 cathode material, thickness expanded monotonically during charging and contracted monotonically during discharge. For the higher-nickel NCM622 cathode material, swelling and contraction were non-monotonic: the cell expanded during most of charging but showed contraction in the high-voltage region near the end of charge. This behavior reversed during discharge — expansion occurred first, then contraction. Across three charge-discharge cycles, NCM622 consistently reproduced this “M”-type swelling pattern, indicating the behavior is intrinsic to high-nickel cathode materials rather than a measurement artifact.

Figure 1.(a) Silicon-based anode swelling in-situ rapidscreening system (RSS1400); (b) NCM111 and NCM622 cathode materials assembled into a button-type full battery, and monitoring the swelling thickness change during three cycles of charge and discharge, Among them, NCM622 exhibits an “M” type of swelling behavior.

3. What Is the Crystal Structure of Layered NCM Cathode Materials?

The NCM cathode electrode belongs to the α-NaFeO2 type crystal ^[1], and its specific crystal structure is shown in Figure 2, where the green is lithium-ion, the blue is transition element (TM) ions, and the red is oxygen ions. The layered units composed of oxygen ions and transition element ions are arranged longitudinally along the c-axis, while lithium-ion are alternately distributed between these layered units along the c-axis, forming a typical ABC-type stacked cubic stacking structure ^[1].

Figure 2. Schematic diagram of the crystal structure of layered LiNixCoyMnzO2^[1].

4. How Does Nickel Content Affect the Swelling Curve of Ternary Cathode Materials?

F.B. Spingler et al.^[2] studied swelling behavior across NCM cathode materials with different nickel contents, plus NCA cathode material, with results shown in Figure 3. As delithiation deepens in NCM111, the swelling curve is relatively flat at first — even showing a slight decline — before trending clearly upward; on discharge lithium intercalation, the curve shrinks noticeably at first, then flattens. As nickel content increases, cathode swelling decreases during delithiation and even contracts at the end of delithiation (the high-voltage region); this contraction is reversible on lithium insertion during discharge, as shown in detail in Figure 3(c) and (d).

Figure 3. Variation of the swelling thickness of the ternary cathode with the gram capacity^[2] (The upper limit cut-off voltage is 4.3V), where (a) is NCM111, (b) is NCM622, (c) is NCM811, (d) is NCA cathode.

Swelling Behavior Comparison Across Ternary Cathode Nickel Content

5. What Does In-Situ XRD Reveal About NCM Cathode Structural Changes During Charging?

To explain this Ni-content-dependent swelling behavior, L.D. Biasi et al. [1] used in-situ XRD to study the 2θ angle shift of the 003 crystal plane for ternary cathode materials with different nickel contents (NCM111, NCM523, NCM622, NCM721, NCM811, and NCM851005) during charging, with results shown in Figure 4. As nickel content increases, the 003 crystal plane shifts to a higher angle under high voltage, indicating the 003 plane spacing shrinks significantly at high voltage.

Figure 4. The change of the 2θ angle of the 003 crystal plane during charging of NCM cathode cells with different Ni contents under in-situ XRD test ^[1].

L.D. Biasi et al. ^[1] then analyzed how the a-axis and c-axis of the NCM crystal vary with voltage (Figure 5). During charging and delithiation, the a-axis contracts first, then flattens; the c-axis expands noticeably at first, then begins to shrink, and as nickel content increases, this second-half c-axis shrinkage becomes more pronounced and its onset voltage moves earlier.

Figure 5. The relative spacing of the a-axis and c-axis with voltage during the charging and delithiation process of NCM cathode cells with different Ni contents ^[1].

The smaller a-axis spacing is generally attributed to oxidation of the transition metal (TM), while the larger c-axis spacing reflects increased Coulomb repulsion between NCM crystal layers after lithium-ion extraction. As delithiation deepens, the c-axis generates a large number of voids — especially in high-nickel ternary materials — eventually causing structural shrinkage (smaller spacing at high voltage). F.B. Spingler et al. [2] note that microscopic swelling accumulates into macroscopic reversible swelling, but microscopic shrinkage does not necessarily produce macroscopic shrinkage — it instead increases certain gaps within the electrode structure. The c-axis expansion and contraction are the main drivers of ternary material swelling and contraction during charging, and as nickel content rises, the c-axis shrinkage voltage advances from about 4.2V to about 4.0V (vs. Li). In a ternary-graphite full-cell system with a typical 3V–4.2V charge window, low-nickel ternary cells expand monotonically on charge and contract monotonically on discharge, while high-nickel ternary cells expand then contract on charge, and expand then contract on discharge — producing the “M”-shaped swelling curve.

6. Why Does LCO Cathode Material Swell Monotonically Compared to High-Nickel NCM?

Lithium cobalt oxide (LCO) also belongs to the α-NaFeO2-type crystal family. B. Rieger et al. ^[3] used in-situ XRD combined with a swelling test system to study LCO cathode swelling behavior during charging (Figure 6). As with NCM, the a-axis contracts during charging and delithiation (Figure 6(a)), while the c-axis shows clear swelling from increased Coulomb repulsion, ultimately producing macroscopic swelling of the LCO crystal. Across the full voltage range, LCO cathode material shows a monotonous swelling trend with no volume shrinkage at high voltage — consistent with low-nickel ternary behavior, since LCO contains no nickel.

Figure 6. (a) When the O3 I phase changes to the O3 II phase, the a-axis, c-axis and unit cell volume change with the charging capacity; (b) During the charging process, the volume of the O3 I phase, the volume of the O3 II phase, and the thickness of the electrode sheet vary with the charging capacity.

7. Summary: What Causes “M”-Type Swelling in High-Nickel Ternary Cathode Materials?

In this paper, using IEST RSS1400 silicon-based anode swelling in-situ rapid screening system, swelling tests on ternary cathode electrodes showed that high-nickel ternary systems exhibit “M”-type swelling at high voltage — a behavior driven by three related mechanisms:

• Coulomb repulsion drives c-axis expansion. Whether LCO or NCM cathode material, the macroscopic structure expands as c-axis Coulomb repulsion increases during charging delithiation.
• Void formation drives late-stage c-axis contraction. At high nickel content, deeper delithiation creates more voids along the c-axis, causing overall structural shrinkage rather than continued expansion.
• The contraction onset voltage advances with nickel content. As nickel content rises, the c-axis shrinkage transition voltage moves earlier — from around 4.2V toward 4.0V — so within a typical 3V–4.2V full-cell charge window, high-nickel ternary full cells contract near the end of charge and re-expand reversibly at the start of discharge, producing the overall “M”-type swelling curve.

8. References

[1] L.D. Biasi, A.O. Kondrakov, H. Gebwein, T. Brezesinski, P. Hartmann and J. Janek, Between Scylla and Charybdis: Balancing Among Structural Stability and Energy Density of Layered NCM Cathode Materials for Advanced Lithium-ion Batteries. J. Phys. Chem. C 121 (2017) 26163–26171.

[2] F.B. Spingler, S. Kucher, R. Phillips, E. Moyassari and A. Jossen, Electrochemically Stable In Situ Dilatometry of NCM, NCA and Graphite Electrodes for Lithium-ion Cells Compared to XRD Measurements. J. Electrochem. Soc. 168 (2021) 040515.

[3] B. Rieger, S. Schlueter, S.V. Erhard and A. Jossen, Strain Propagation in Lithium-ion Batteries from the Crystal Structure to the Electrode Level. J. Electrochem. Soc. 163 (2016) A1595-A1606.

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