Battery electric vehicles are considered a key technology for achieving zero-emission mobility, increasing the demand for efficient battery thermal management systems (BTMSs) capable of ensuring both high performance and long battery lifetime. While conventional BTMS concepts effectively control the overall battery temperature, unavoidable coolant temperature increase along the flow path results in temperature gradients across battery modules. These temperature differences cause inhomogeneous aging of lithium-ion battery cells and can significantly affect long-term module performance and life-cycle characteristics. This work investigates the impact of BTMS-related thermal inhomogeneities on battery degradation and develops a holistic electro-thermal-fluid-aging modeling framework for accurate life-cycle analysis of lithium-ion battery modules. The results demonstrate significant aging differences for conventional cooling concepts and motivate the development of a novel switchable BTMS architecture. By periodically reversing the coolant flow direction between load cycles, the proposed concept reduces thermal aging inhomogeneities and increases total charge throughput and vehicle mileage by up to 7.2 %.