Rechargeable　metal　batteries　have　received　widespread　attention　due　to　their　high　energy　density　by　using　pure　metal　as　the　anode.　However,　there　are　still　many　fundamental　problems　that　need　to　be　solved　before　approaching　practical　applications.　The　critical　ones　are　low　charge/discharge　current　due　to　slow　ion　transport,　short　cycle　lifetime　due　to　poor　anode/cathode　stability,　and　unsatisfied　battery　safety.　To　tackle　these　problems,　various　strategies　have　been　suggested.　Among　them,　electrolyte　additive　is　one　of　the　most　widely　used　strategies.　Most　of　the　additives　currently　studied　are　soluble,　but　their　reliability　is　questionable,　and　they　can　easily　affect　the　electrochemical　process,　causing　unwanted　battery　performance　decline.　On　the　contrary,　insoluble　additives　with　excellent　chemical　stability,　high　mechanical　strength,　and　dimensional　tunability　have　attracted　considerable　research　exploration　recently.　However,　there　is　no　timely　review　on　insoluble　additives　in　metal　batteries　yet.　This　review　summarizes　various　functions　of　insoluble　additives:　ion　transport　modulation,　metal　anode　protection,　cathode　amelioration,　as　well　as　battery　safety　enhancement.　Future　research　directions　and　challenges　for　insoluble　solid　additives　are　also　proposed.　It　is　expected　this　review　will　stimulate　inspiration　and　arouse　extensive　studies　on　further　improvement　in　the　overall　performance　of　metal　batteries.　©　2024　The　Authors.　Small　published　by　Wiley-VCH　GmbH.