Investigating Differences in Battery Health Among Semi-Solid State Lithium-Ion Cells with Similar Capacity

Abstract

Lithium-ion batteries are widely used in electronics and electric vehicles due to high energy density, long cycle life, and rechargeability. As battery use continues to expand, accurately evaluating battery health has become increasingly important. However, battery capacity alone does not always reflect how well a battery performs or how much it has degraded. This study investigated whether commercially available semi-solid state lithium-ion cells with nearly identical capacities could still differ in electrical performance. The cells were tested using repeated charge-discharge cycling and electrochemical impedance spectroscopy (EIS). During testing, voltage, current, energy, internal resistance, temperature, and voltage relaxation were measured and compared. Although the two cells had nearly identical initial capacities (3057 mAh and 3058 mAh), one exhibited approximately 40% higher internal resistance (56.4 mΩ versus 40.3 mΩ), consistently lower operating voltage by 10 to 20 mV, lower discharge energy (10.830 Wh versus 10.879 Wh), lower round-trip energy efficiency (91.1% versus 92.4%), and less voltage recovery after discharge (130 mV versus 160 mV). These results show that batteries with nearly identical capacities can still differ substantially in power delivery and overall performance. The findings demonstrate that evaluating battery health requires more than capacity measurements alone and highlight the value of complementary diagnostic techniques such as impedance and resistance measurements.