61 E-Mobility Engineering | May/June 2024 and discharge. The cell is designed to dissipate excess heat during fast charging and deliver good packaging efficiency to achieve higher cell-level energy density. Temperature and pressure All batteries generate a certain amount of heat during operation, and higher rates of power typically result in more heat. This means that to enable vehicles with quick acceleration and fast charging, you need a BMS that can quickly pull heat out of the individual battery cells to keep them at their optimal operating temperature. In this context, a high-performance battery cell not only needs good fundamental chemistry, but also intelligent cell design to enable good thermal performance. Conventional cell designs all manage heat in more or less the same way: as the cell heats up during operation, the thermal energy must move from the layers of the cell to the external casing, where the pack-cooling system draws it away. However, a lithium-metal cell format must expand and contract, and it is difficult to engineer a mechanism, such as a spring or foam, which can move flexibly while conducting heat effectively from the cell face. To address this challenge, the FlexFrame design splits these tasks in two. While the exterior face of the cell can expand and contract as necessary, the heat generated can be transferred directly from the individual cell layers to the exterior frame, which can then be cooled from the back or sides. Thermal management systems incorporating the FlexFrame cell design are currently being developed and validated. FlexFrame is designed to work with or without external pressure. If required, pressure can be applied to the external face of the cell, and because heat can be transferred out to the sides of the frame, it is possible to apply pressure externally while cooling the cell at the same time. Data from testing the single-layer cells has shown that the solid-state lithiummetal cell can deliver a long cycle life with zero externally applied pressure, and FlexFrame is intended to be able to operate under these conditions as well. Manufacturing and pack integration As a hybrid of conventional prismatic and pouch designs, FlexFrame fits into the existing manufacturing flow. The exterior polymer laminate material is similar to what is found in conventional, lithium-ion pouch cells, and the frame gives the cell good mechanical stability, which is provided at the module level for a conventional pouch cell. Assembly of the cell uses conventional stacking and sealing a conventional pouch cell and a prismatic cell, and it is intended to enable a step-change improvement through the simplicity of design. There are two fundamental features of the FlexFrame architecture: a frame that wraps around the edge of the cell stack, and a flexible outer layer of polymer laminate, similar to conventional pouchcell material. When manufactured, the cell is anode-free, and the cell stack is in its most contracted position, with the face of the cell sitting about 1 mm below the frame. As the cell charges and the anodes of each layer are plated with pure lithium metal, the cell face is pushed out, along with the flexible packaging material. When fully charged, the face of the cell is designed to be almost totally flush with the frame. In addition to accommodating expansion and contraction, FlexFrame is designed to allow the cell to simultaneously dissipate excess heat during fast charging and function, with or without externally applied pressure. This offers good packaging efficiency to achieve cell-level energy density targets. The cell has a frame that wraps around the edge of the cell stack and a flexible, polymer outer layer, and it is designed to accommodate the uniaxial expansion and contraction that lithium-metal batteries experience during charge Detailed simulation of the battery pack can improve energy density (Image courtesy of About:Energy) Modelling the Molicel lithium-ion cell (Image courtesy of About:Energy)
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