You will be shown an abstract from a biomedical research paper. Given this abstract, your task is to extract all unique entities of the following types: ["Chemical"].

Please return the output as a JSON object of the format: {"Chemical": ["Arsenic", ...]}. The keys should be entity types and values should be lists of extracted entities belonging to the corresponding type. If you cannot find entities belonging to a specific type, the value should be [].

Only output the JSON object and do not include any additional text.

Abstract:

A rechargeable room-temperature sodium superoxide (NaO2) battery. In the search for room-temperature batteries with high energy densities, rechargeable metal-air (more precisely metal-oxygen) batteries are considered as particularly attractive owing to the simplicity of the underlying cell reaction at first glance. Atmospheric oxygen is used to form oxides during discharging, which-ideally-decompose reversibly during charging. Much work has been focused on aprotic Li-O(2) cells (mostly with carbonate-based electrolytes and Li(2)O(2) as a potential discharge product), where large overpotentials are observed and a complex cell chemistry is found. In fact, recent studies evidence that Li-O(2) cells suffer from irreversible electrolyte decomposition during cycling. Here we report on a Na-O(2) cell reversibly discharging/charging at very low overpotentials (< 200 mV) and current densities as high as 0.2 mA cm(-2) using a pure carbon cathode without an added catalyst. Crystalline sodium superoxide (NaO(2)) forms in a one-electron transfer step as a solid discharge product. This work demonstrates that substitution of lithium by sodium may offer an unexpected route towards rechargeable metal-air batteries.
{"Chemical": ["oxides", "sodium", "carbon", "Li(2)O(2)", "NaO2", "Li-O(2)", "NaO(2)", "lithium", "sodium superoxide", "carbonate", "oxygen", "Na-O(2)"]}