Utilizing a custom developed chamber, the internal pressure evolution of a commercial 2.6 Ah, LiFePO 4 //Graphite lithium-ion battery (LIB) was studied throughout elevated rate lifecycle testing. Verification tests confirmed the custom test chamber and puncture procedure resulted in minimal changes to the LIB performance. Capacity measurements and analysis confirmed the battery's accessible capacity faded 20% after 500 cycles as a result of the increase in DC Equivalent Series Resistance (ESR). Electrochemical Impedance Spectroscopy (EIS) measurements and modeling analysis also established that minimal material change occurred within the LIB during lifecycle testing and the decreased capacity is believed to primarily result from the increase in battery ESR. Analysis of the battery's internal pressure evolution during the baseline discharges yielded information elucidating apparent electrode volume changes during the 1 C discharge. A strong statistically significant correlation was identified between the 1C molar density delta and the battery capacity fade. Differential capacity analysis was used to examine the correlation of the behavior with the de-intercalation process. The pressure data and molar density analysis indicated a reduction in the amount of lithium de-intercalated during discharge as a result of the rise in ESR and thus a measurable shift in the volume as the LIB ages.Over the last two decades, the demand for energy dense electrochemical storage devices has grown exponentially in response to the proliferation of portable electronics. This demand has expanded beyond energy density to include the need for power density as applications such as hybrid electric vehicles, full electric vehicles, aircraft backup power, grid-tie energy storage, and renewable energy emerge. While there are many electrochemical storage devices that work well in either energy or power applications, LIBs have led the way in combining both energy and power density making them the chemistry of choice for these demanding applications. These storage systems are typically driven by performance, cost, and safety. Due to these driving factors, an innovative evaluation of LIB life and failure prediction is needed. The complexity of LIBs creates a challenge to this evaluation and an investigation of the aging process further complicates the pursuit. A comprehensive review of the mechanisms of aging in LIBs was conducted for the FreedomCAR research initiative, and reports that the 'Capacity decrease and power fading do not originate from one single cause, but from a number of various processes and their interactions. Moreover, most of these processes cannot be studied independently and occur at similar timescales, complicating the investigation of aging mechanisms. 1 Numerous studies have examined the causes for capacity fade in LIBs and the result indicates that capacity fade is linked to chemical and structural changes within the battery. Electrolyte decomposition occurs contributing to the growth of passivation layers at the electrode...