A high sensitivity thermoelectric sensor to measure all relevant thermal transport properties has been developed. This so-called transient hot bridge (THB) decidedly improves the state of the art for transient measurements of the thermal conductivity, thermal diffusivity, and volumetric specific heat. The new sensor is realized as a printed circuit foil of nickel between two polyimide sheets. Its layout consists of four identical strips arranged in parallel and connected for an equal-ratio Wheatstone bridge. At uniform temperature, the bridge is inherently balanced, i.e., no nulling is required prior to a run. An electric current makes the unequally spaced strips establish an inhomogeneous temperature profile that turns the bridge into an unbalanced condition. From then on, the THB produces an offset-free output signal of high sensitivity as a measure of the properties mentioned of the surrounding specimen. The signal is virtually free of thermal emf's because no external bridge resistors are needed. Each single strip is meander-shaped to give it a higher resistivity and, additionally, segmented into a long and short part to compensate for the end effect. The THB closely meets the specific requirements of industry and research institutes for an easy to handle and accurate low cost sensor. As the key component of an instrument, it allows rapid thermal-conductivity measurements on solid and fluid specimens from 0.02 to 100 W· m −1 ·K −1 at temperatures up to 250 • C. Measurements on some reference materials and thermal insulations are presented. These verify the preliminary estimated uncertainty of 2% in thermal conductivity.

Nail penetration tests under adiabatic conditions are proposed as a new safety test method to simulate a short circuit caused by intrusion in the worst case scenario, i.e., adiabatic conditions. The behaviors of commercially available 18650 type lithium-ion cells with two different cell chemistries, LiNi 0.33 Co 0.33 Mn 0.33 O 2 -LiCoO 2 blend (NCM-LCO blend) vs. graphite and LiFePO 4 (LFP) vs. graphite, were investigated at different states of charge (SoC). The results show a small standard deviation for the temperature and pressure measurements from five identical experiments. The detailed examination of the nail penetration experiments reveals differences in the safety properties of the cathode materials (LFP vs. NCM-LCO), separator types (PP PE PP trilayer vs. PE single layer) and electrode designs (high power vs. high energy). For both cell types, the observed cooling effect at high energy contents could be linked to an increased gas production due to the loss of electrolyte.

Investigations on the effect of flame retardant additives (FRs) on the abuse tolerance of large scale lithium ion battery (LIB) cells (5 Ah) are of high relevance in battery science and industry but rarely performed as they are cost and time consuming. In addition, even though FRs are frequently investigated, their positive effect on the safety properties of larger full LIB cells under abusive condition has not been proven yet. The promising FR (phenoxy) pentafluorocyclotriphosphazene (FPPN) is known to exhibit excellent flame retardant-and electrochemical properties at the same time.Therefore, FPPN is investigated towards abuse tolerance in 5 Ah LIB cells in this study. Calorimetric investigations show that a mass percentage of 5 wt % FPPN mixed to a standard electrolyte, significantly reduces the self-heating rate of 5 Ah cells in the temperature range from 80°C to 110°C. While nail penetration and external short circuit experiments provide no significant difference between standard and FPPN-containing cells, an increased overcharge tolerance and a favorable thermal stability at � 120°C in overcharge and oven experiments could be shown. [a] T. . Nail penetration experiment of 5 Ah cells containing electrolyte with (blue) and without (black) 5 wt % FPPN. a) scheme showing the position of the thermocouples. b) Voltage over time of two cells per additive mixture. Temperature over time of two cells per electrolyte mixture are presented in c) for thermocouple position 1 and in d) for thermocouple position 2. . Overcharge experiment of 5 Ah cells containing electrolyte with (blue) and without (black) 5 wt % FPPN. a) Voltage and b) temperature (position 1) vs. time diagrams of two cells per additive mixture are presented.

The thermal runaway behavior during a nail penetration test of lithium metal oxide based commercially available 18650-type lithium ion cells was investigated regarding the influence of nail design, state of charge (SOC) and state of health (SOH) under quasi-adiabatic conditions. Whether an immediate or a delayed thermal runaway occurs, is highly dependent on the angle of the nail tip. A rather blunt nail provokes a more severe reaction leading to an immediate thermal runaway at 100% SOC. The lower the SOC, the less severe is the cell self-heating. After nail penetration further self-heating causing a thermal runaway is highly dependent on the adjusted SOC. The impacts of aging result to be beneficial for safety down to a SOH of 90%.

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